The present invention relates to a linear motion device used in, for example, the machine tool. More particularly, the present invention relates to a linear motion guide bearing device in which retainer pieces are each interposed between adjacent rolling elements which circulatingly roll in the longitudinal direction of a guide rail. In addition, the present invention relates to a ball screw device in which retainer pieces are each interposed between adjacent rolling elements which circulatingly roll along an element rolling groove, spirally shaped, defined by a thread groove of a ball screw shaft and a thread groove of a nut.
The present invention relates to retainer pieces each disposed between adjacent balls in order to secure good operation of a ball screw. In this specification, the term xe2x80x9cthe thickness of the retainer piecexe2x80x9d or the xe2x80x9cthicknessxe2x80x9d on the retainer piece does not means the thickness of the whole retainer piece, but means the thickness of the retainer piece to separate the surface of a ball from the surface of another ball adjacent to the former. To be more precise, the xe2x80x9cthicknessxe2x80x9d on the retainer piece means that the thickness of the retainer piece at a position with respect to a position defined a line connected centers of adjacent rolling elements, when two rolling elements are arranged on a collinear.
A linear motion guide bearing device 10 as shown in FIG. 9 is known for a device for linearly guiding a work table of, for example, a machine tool. The linear motion guide bearing device 10 is provided with a guide rail 11 being rectangular in cross section. Rolling grooves 12 for the rolling elements are formed on both side surfaces of the guide rail 11, and from one end to the other end of the guide rail 11. A plurality of spherical rolling elements 13 (see FIG. 10) are engaged with each other in the rolling grooves 12. When the rolling elements 13 roll along the rolling element grooves 12, a slider 14 is relatively moved in the longitudinal direction of the guide rail 11.
In the slider 14 have a slider body 15 straddling the guide rail 11, and end caps 16 provided at the front end and the rear end of the slider body 15. Rolling grooves 17 for the rolling elements (see FIG. 10) are arcuate in cross section and are formed in the inner side walls of the each of sleeves 15a of the slider body 15 of the slider 14, respectively. The spherical rolling elements 13 roll in and along a rolling-element passage which is formed between the rolling grooves 17 and the rolling grooves 12.
A passage hole for the rolling elements 18 is provided within each of the sleeves 15a of the slider body 15 and extends parallel to the rolling grooves 17. The rolling grooves 17 and the passage holes 18 form an endless circulating raceway 20 of the rolling elements 13 together with rolling-element return passages 19 formed in the end caps 16.
In the linear motion guide bearing device, when the rolling elements forcibly rub with each other with the movement of the slider 14, early wear of the rolling elements and generating noise are frequently caused. To avoid this, retainer pieces 21 are each interposed between adjacent rolling elements 13, thereby preventing the rolling elements from coming in contact with one another. In the linear motion guide bearing device, as shown in FIG. 11, the pitches among the rolling elements of a rolling element series 22 which is constructed with spherical rolling elements 13 and the retainer pieces 21 are equal in dimension. Accordingly, the periodical passing vibration of the rolling elements occurs at a fixed period with the movement of the slider 14. The periodical passing vibration vibrates the bearing. This makes it difficult to further enhance the noise characteristic and motion accuracy. The same thing is correspondingly applied to the screw device in which the retainer pieces are each interposed between adjacent rolling elements.
In Japanese patent application No. Hei. 9-100839 (U.S. Pat. No. 5,927,858 (Jul. 27, 1999), a linear motion guide bearing device in which thereby eliminating the presence of indentations on the raceway surface on which the rolling elements 13 roll, or reducing the vibrations and noises, which are due to the fact that the rolling elements 13 on the element endless circulating raceways simultaneously enter the load path is described.
In the linear motion guide bearing device disclosed, the retainer pieces, which are each interposed between adjacent rolling elements in the rolling element series, are all different in their pieces having different thicknesses must be prepared. This brings about cost increase in the manufacturing of the retainer pieces and assembling of them into between the adjacent rolling elements.
In a ball screw device, balls(rolling elements) are arranged in the ball screw to support a load. Those balls roll in a circulating manner, and through the ball rolling, a good rotation-to-linear motion conversion is secured while maintaining load capacity and rigidity. Since the rolling of the balls occurs randomly, sometimes adjacent balls act to mutually impart compression forces to each other, and the slipping state often occurs. As a result, the balls exhibit great resistance to impede the rolling of other balls, so that a torque variation occurs and causes operation trouble.
A ball screw is known which, to solve such a problem, retainer pieces are each disposed between adjacent balls, and resistance acting to impede the rolling of other balls is not generated by avoiding the mutual ball contact.
In the ball screw, the balls are arranged at substantially regular spatial intervals by use of the retainer pieces, and the following new problems arise.
1) As shown in FIG. 12, when the surface 62 of the shaft on which balls 61 are disposed or the groove surface of the nut, which surface is brought into contact with the balls 61, is ground, very fine process-waving 63 caused by the oscillation of a grinding wheel inevitably takes place on the contacting surface. This is a phenomenon always occurring in machining objects as well as in the ball screw. This fact implies that even if an object is highly precisely machined and the precision of the machined object is visually (macroscopically) high, the very fine process-waving occurs when microscopically observed. In this state, retainer pieces 64 having thicknesses being uniform in value are disposed in the ball screw. As a result, there is a case that a distance 66 between adjacent balls of those regularly arranged balls is coincident or substantially coincident in length with the pitch of the process-waving. If those are coincident, the number of contact positions 68 where the balls come in contact with top parts of the fine process-waving is equal to the number of the balls as the greatest number. Also when the distance is coincident with the bottom parts of the fine process-waving, the number of contact positions is equal to the number of the balls as the greatest number. The influence by the variation of friction acts by the number of balls. This possibly leads to operation impairment due to the friction variation, and abnormal sound generation and noise increase, which result from the coincidence of the balls 61 with the process-waving 63.
2) Where in the ball screw utilizing the retainer pieces 64, one kind of retainer pieces is used and the machining accuracy is not different, to avoid the problem 1) above arising from the coincidence of the ball(element)-to-ball(element) distance between the balls with the pitch of the process-waving 63, consideration at design or machining stage is needed, so that the pitch of the very fine process-waving 63 is not coincident with the ball-to-ball distance. In other words, consideration on a microscopic condition of the surface produced by the machining is an essential matter in design and machining. This is very difficult and needs complicated work.
3) Also in controlling, in design, the number of balls 61 and the number (referred to as a filling factor) of the retainer pieces 64 to a length (referred to as a circuit length) of a space in which the balls 61 are put, in the case of using one kind of retainer pieces 64, balls 61 are arranged at regular intervals 66 as shown in FIG. 13. As a result, a space 65 having a size being incapable of receiving a new ball 1 is left sometimes. If such a space is present, there is a chance that the retainer piece (indicated by a wavy line 67 in the figure) falls when the balls 61 roll.
Accordingly, an object of the present invention is to provide a linear motion device such as a linear motion guide bearing device and a ball screw, which reduce the vibrations and noises, which are ascribed to the periodical passing vibration of rolling elements, without increasing the manufacturing cost.
To achieve the above object, a first aspect of the present invention provides a linear motion guide bearing device comprising: a guide rail having rolling grooves extending in an axial direction of the linear motion guide bearing device and formed on the outer surface thereof; a slider engaged with the guide rail so as to move in the axial direction of the guide rail; a plurality of rolling elements interposed between the slider and the rolling grooves of the guide rail; and a plurality of retainer pieces each interposed between adjacent rolling elements, the retainer pieces including a plurality kinds of retainer pieces having different thicknesses, and the number of kinds of the retainer pieces is smaller than the number of the rolling elements of the rolling element series.
In addition to this, a second aspect of the present invention provides a ball screw device comprising: a screw shaft having a first thread groove spirally formed in an outer peripheral surface thereof; a nut having a second thread groove in the inner peripheral surface thereof and being fitted to the screw shaft, the second thread groove corresponding to the first thread groove of the screw shaft; a plurality of rolling elements rollably received in a space defined between the first and second thread grooves; a circulating member fixed to the nut so as to form a circulating passage through which the rolling elements rolling along the space defined between the first and second thread grooves endlessly circulate; and a plurality of retainer pieces each interposed between adjacent rolling elements, the retainer pieces including a plurality kinds of retainer pieces having different thicknesses, and the number of kinds of the retainer pieces are smaller than the number of the rolling elements of the rolling element series.
With such a mechanical arrangement, there is no need of preparing a plurality of different kinds of retainer pieces having different thicknesses. Therefore, the linear motion device is capable of reducing the vibrations and noises, which are ascribed to the periodical passing vibration of the rolling elements, without increasing the cost.
In this case, when the preparation of retainer pieces and the assembling of the retainer pieces to between the rolling elements are allowed for, it is preferable that the retainer pieces are two to five number of kinds of retainer pieces. If the number of retainer pieces is so selected, the retainer pieces preparation is well balanced with the retainer pieces assembling. When the rolling element series is formed using different kinds of retainer pieces having different thicknesses, it is preferable that every kinds of retainer pieces having different thicknesses are sorted by colors, and each retainer piece has an identifying mark every kind of retainer piece. This feature accrues to the following advantages. It is prevented that the retainer pieces are each assembled to between adjacent rolling elements erroneously. The management and discrimination of the retainer pieces are easy.
Some significant difference must be allowed to be present in the element-to-element pitches among the different kinds of retainer pieces. When allowing for the durability of the retainer pieces, the dimension of the radius of curvature of the circulating raceway, and the minimizing of the load capacitance, the significant difference of the element-to-element pitches is preferably 1% to 10% of the diameter of the rolling element.
Further it is preferred that the retainer pieces have each an elastic structure or are each made of elastic material.
It is preferred that a method of controlling a distance of the center-to-center distance between the adjacent balls or the number of the balls by making different the thicknesses of the retainer pieces each located between the adjacent balls.
1) In reference to FIGS. 12 and 13, even in a case where the pitch of the process-waving 63 is coincident with the distance between adjacent balls 61 (the ball-to-ball distance), the ball-to-ball distance may easily be changed by arranging retainer pieces 64 having different thicknesses (at least two kinds of retainer pieces). As a result, the pitch of the process-waving 63 is not coincident in length with the ball-to-ball distance. Accordingly, the ball screw of the present invention is free from the synergy based on the number of balls, which results from the non-coincidence of the pitch of the process-waving 63 with the ball-to-ball distance, and succeeds in solving the operation impairment and noise problem, when comparing with the case where the ball-to-ball distance cannot be changed.
2) Freedom of selection of the retainer pieces 64 is large. Accordingly, after the ball-to-ball distance is incorporated into a design condition, there is no need of considering the process-waving 63 and the ball pitch in the stage of design or machining. Design or machining work is lessened leading to labor saving.
3) Freedom of selecting the thicknesses of the retainer pieces 64 is large. Accordingly, the filling factor of balls is easily controlled by changing the retainer pieces 64. With this, in the initial stage of design, design work may proceed without taking the circuit length and the filling factor into consideration.
In addition to the above-mentioned effects, the following effects may also be produced.
4) Since the freedom of selection of retainer pieces is large, the secondary effect allowing a designer to select the retainer piece having such a thickness as to reject its coincident with the pitch of the process-waving is produced in addition to the feature of the present invention that the retainer pieces to be arranged are not uniform in thickness value.